The present invention relates to extensometers in general, and more particularly to improvements in so-called rod extensometers which can be used with advantage to monitor eventual shifting of strata of rock, ore or other material surrounding a tunnel or another underground excavation.
Certain types of presently known rod extensometers are described, for example, in Prospektblatt Interfels published in 1970 by Internationale Versuchsanstalt fuer Fels in Salzburg, Austria, Datenblatt 2.5, sheets 1, 3, 3a and 3b. Such extensometers are particularly suited for precise measurements of minute shifting of strata of soil, rock ore or like material around a tunnel or another underground excavation. A drawback of presently known extensometers is that they are rather bulky, complex and expensive. Also, presently known rod extensometers comprise a large number of component parts and few of such parts can be recovered upon completion of measurement in a particular region of the excavation. Still further, owing to the bulk of known extensometers, they can only be installed in holes having relatively large diameters; the drilling of such holes in rock or ore is expensive and time-consuming since the length of a hole may be in the range of several hundred meters.
Systematic monitoring of movements of material around an underground excavation is important and desirable in coal mines and similar plants. Such monitoring is necessary to determine the magnitude of stresses which are applied by surrounding material to the frame structure or lining in the excavation, especially in an excavation which is to be used for a relatively short period of time and is dug into material exhibiting a strong tendency to shifting. There exists an urgent need for extensometers which are relatively inexpensive so that they can be used in groups of two or more and which can be rapidly installed in or removed from holes drilled into material around an underground excavation in a coal mine or the like.